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\begin{document}

\title{A Wearable Active GPS Antenna For Application in Smart Textiles} %!PN

\author{Timothy De Keulenaer, Arnaut Dierck \thanks{T.~De Keulenaer and A.~Dierck are with the Electrical
    Engineering Department, Ghent University (UGent), Gent,
Belgium.\newline E-mail: timothy.dekeulenaer@intec.ugent.be, arnaut.dierck@intec.ugent.be .}}

\supervisor{Frederick Declercq, prof. dr. ir. Hendrik Rogier, dr. ir. Dries Van De Ginste}

\maketitle
\begin{abstract}
This paper describes the design of a wearable, active Global Positioning System antenna for application in smart textiles. The antenna is a circularly polarized aperture coupled microstrip patch antenna for use in the L1-band. A quadrature hybrid coupler was used to obtain circular polarization. A low noise amplifier was integrated on the backside of the passive radiator. An antenna gain of 5 dBi was measured, which, together with the active gain of 18.1 dB, results in a measured total gain of 23.6 dB.
\end{abstract}
\begin{keywords}
aperture coupled microstrip patch antenna, wearable, circular polarization, active, GPS, smart textiles, LNA
\end{keywords}
\section{Introduction}
\PARstart{F}{or} space, aeronautical and terrestrial communications, there is a need for low-weight, flexible Smart Fabrics, Interactive
Textile (SFIT) electronic systems that are unobtrusively integrated into garments, without disturbing the comfort or the
movements of the wearer. Given its size, the wearable antenna forms a vital component in such a system. The last decades,
research on textile antennas has boomed and many robust textile antennas have been proposed that exhibit
efficient radiation characteristics under many adverse conditions. However, the connection between the
wearable antenna and the transceiver remains a weak link that easily breaks when exposed to extreme movements, such as
performed by rescue workers during an intervention. Therefore, in this contribution a more robust solution is presented by
integrating an active electronic system directly underneath the textile patch antenna and by using aperture coupling as a
feeding technique instead of a coaxial feed.

Specifically, in this contribution an active GPS antenna is presented. A low noise amplifier (LNA) was integrated on the antenna backside, minimizing the coupling between the antenna and the active circuitry. The antenna geometry is presented in Section \ref{sec:geom}. The design of the LNA is discussed in Section \ref{sec:LNA}. Section \ref{sec:meas} presents the measured antenna and LNA characteristics.

\section{Antenna geometry and design}
\label{sec:geom}
\subsection{Material selection}
As shown in Figure \ref{fig:antenna_crosssection}, the active aperture coupled patch antenna consists of an antenna substrate and a feed substrate. The characteristics of the substrates are shown in Table \ref{tab:char}. The antenna patch, feed lines and ground plane with slots are etched in a flexible copper-on-polyimide film.
\begin{figure}[h]
	\centering
		\includegraphics[width=0.35\textwidth]{figuren/antenna_crosssection}
	\caption{Geometry of the GPS antenna}
	\label{fig:antenna_crosssection}
\end{figure}
\begin{table}[htb]
\caption{Properties of feed and antenna substrate}
\label{tab:char}
\begin{center}
{\tt
\begin{tabular}{|c||c|c|}\hline
Substrate&Feed&Antenna\\\hline\hline
Material&aramid&polyurethane\\\hline
$\epsilon_{r}$&1.99&1.25\\\hline
tan$\delta$&0.016&0.015\\\hline
h&0.40 mm&3.56 mm\\\hline
\end{tabular}
}
\end{center}
\vspace{-15pt}
\end{table}

\subsection{Antenna geometry}
The realized antenna geometry consists of a square patch, two slots and a feed line for each slot forming a 90$^\circ$ bend to accomodate the quadrature hybrid coupler, as shown in Figure \ref{fig:final_antenna}. Each slot is aligned to the middle of the edge on which it creates a radiating mode. The center position of the slots allows for a strong excitation of two orthogonal modes on the patch, whereas the quadrature hybrid coupler accomodates a 90$^\circ$ phase difference between the modes over a wide frequency range.
\begin{figure}[h]
	\centering
		\includegraphics[width=0.3\textwidth]{figuren/final_antenna.pdf}
	\caption{Geometry of the GPS antenna}
	\label{fig:final_antenna}
\end{figure}
\subsection{Antenna design}
The antenna design was carried out by imposing criteria for the return loss $|S_{11}|$  and the axial ratio AR. Since the antenna structure is symmetric, only the return loss of one feed line was optimized during design. It is, however, important to ensure that the coupling between the two feed lines is low, so that the power is indeed delivered to the antenna for radiation. The antenna simulations were carried out in the ADS Momentum 2.5D environment. The final antenna dimensions are specified in Table \ref{tab:dim}.
\begin{table}[htb]
\vspace{-15pt}
\vspace{5pt}
\caption{Final antenna dimensions}
\label{tab:dim}
\begin{center}
{\tt
\begin{tabular}{|c|c||c|c|}\hline
Parameter&Size(mm)&Parameter&Size(mm)\\\hline\hline
$L$&75.25&$L_{slot}$&23.00\\\hline
$L_{stub}$&16.80&$W_{slot}$&4.20\\\hline
\end{tabular}
}
\end{center}
\vspace{-15pt}
\end{table}
\section{LNA design}
\label{sec:LNA}
\subsection{LNA topology}
The LNA was built around an Avago ATF-54143 pHEMT transistor, using a grounded-source topology. A 3 V DC voltage can be supplied via the output UFL connector. The LNA layout including active and passive components is shown in Figure \ref{fig:LNA}.
\begin{figure}[h]
	\centering
		\includegraphics[width=0.4\textwidth]{figuren/LNA.pdf}
	\caption{LNA circuit}
	\label{fig:LNA}
\end{figure}
\subsection{LNA design}
The criteria imposed for the LNA design were a high gain, good in- and output matching and, of course, a low noise figure. Since the LNA is placed in-between two 50 $\Omega$ terminated devices, the $|S_{21}|$ can be used as a gain measure. In- and output matching are performed using matching networks, imposing a $<$ -10 dB condition on both the $|S_{11}|$ and $|S_{22}|$. As for the noise figure, we aimed at a value around 0.5 dB. The passive LNA interconnections and transmission lines were simulated in the ADS Momentum 2.5D environment, while the discrete active and passive components were simulated in the ADS circuit simulator.

\section{Measurements}
\label{sec:meas}
The measurements are performed using the N5242 A PNA-X Vector Network Analyzer from Agilent Technologies.
\subsection{Antenna measurements}
The radiation pattern of the passive antenna was measured in both the XZ and the YZ plane, using a standard gain horn in the anechoic chamber. Patterns in both planes are similar. Measurements were also performed for an antenna covered by aramid, to analyse the effect of integration into a textile garment. The aramid layer had no noticeable influence on the antenna characteristics. The XZ-pattern of the aramid covered antenna is shown in Figure \ref{fig:radpat}. The antenna gain at 0$^\circ$ is 5 dBi.
\begin{figure}[h]
	\centering
		\includegraphics[width=0.25\textwidth]{figuren/gainGPS_XZaramide.pdf}
	\caption{Radiation pattern of the aramid covered passive antenna in the XZ plane}
	\label{fig:radpat}
\end{figure}
\subsection{LNA measurements}
The measured LNA properties at 1575 MHz are shown in Table \ref{tab:LNAprops}. For the $IP_{3}$, two tones with a 50 kHz spacing were used. Port 1 of the VNA was connected to the LNA input, port 2 to the output. The 28.0 mA DC current yields a power consumption of 84.0 mW.
\begin{table}[htb]
\caption{Measured LNA properties}
\label{tab:LNAprops}
\begin{center}
{\tt
\begin{tabular}{|c|c||c|c|}\hline
Property&Value&Property&Value\\\hline\hline
$|S_{11}|$&-7.8 dB&$P1dB$&-6.6 dBm\\\hline
$|S_{22}|$&-10.6 dB&$IIP_{3}$&9.5 dBm\\\hline
$|S_{21}|$&18.1 dB&$NF$&0.5 dB\\\hline
$|S_{12}|$&-24.4 dB&$I_{DC}$&28.0 mA\\\hline
\end{tabular}
}
\end{center}
\vspace{-15pt}
\end{table}

\subsection{Active antenna measurements}
The combined gain, consisting of the antenna and LNA gain, in the main radiation direction is 23.6 dB. The measured AR, F/B ratio and 3dB beamwidth of the active antenna at the GPS frequency are 0.7 dB, 19 dB and 70$^\circ$, respectively. The radiation pattern is very similar to the passive antenna radiation pattern.
\section{Conclusions}
The presented active antenna is able to cover the 2.042 MHz wide GPS L1 band at 1575.42 MHz with a combined antenna and LNA gain of 23.6 dB, a noise figure of 0.5 dB, an axial ratio of 0.7 dB and an $|S_{11}|$ of -9 dB. Upon connection of the active antenna to a GPS reciever, it provided us with better carrier to noise ratios than the off-the-shelf active antenna.
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\begin{thebibliography}{1}
\bibitem{avago}
A.J.Ward,
\newblock {AN-1222 High Intercept Noise Amplifier for the 1850 - 1910 MHz PCS Band using the Agilent ATF-54143 Enhancement Mode PHEMT},
\newblock Agilent Technologies, 2006.
\bibitem{artesa}
Dierck A., De Keulenaer T., Declercq F., Rogier H., Ghent University, BELGIUM,
\newblock {A Wearable Active GPS Antenna For Application in Smart Textiles},
\newblock {32nd ESA Antenna Workshop on Antennas for Space Applications, 5-8 October 2010},
\newblock {Submitted}
\bibitem{thesis}
T. De Keulenaer, A. Dierck,
\newblock {Ontwerp van een actieve GPS-antenna voor toepassing in intelligent textiel},
\newblock Master Thesis, 2010.
\end{thebibliography}
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